A typical liquid crystal display device comprises two glass slides spaced 6-12 .mu.m apart enclosing a layer of a liquid crystal material. Electrode structures on the inner face of the slides enable an electric field to be applied across the layer of liquid crystal material.
There are three types or phases of liquid crystal material, cholesteric, nematic, and smectic. This invention concerns cholesteric type material which may use a mixture of natural cholesteric material diluted with nematic material to obtain the desired cholesteric pitch. In a cholesteric material the molecules are arranged in a helical form whose natural pitch can be increased by the addition of a nematic material.
One known type of display utilizes a cholesteric material in a phase change effect. When a suitable electric field is applied to the cholesteric material it causes the molecules to rotate to lie along the applied field, its ON state. This ON state is similar to the ON state of a nematic liquid crystal material and accounts for the device being termed a phase change (cholesteric to nematic ) device. On removing the electric field the liquid crystal relaxes back to its cholesteric, OFF, state.
In its OFF state a phase change device is light scattering. Pleochroic dyes may be dissolved in the liquid crystal material so the OFF state is coloured. In the ON state dye molecules, aligned parallel to the liquid crystal molecules by the known guest host effect, do not absorb light and so the display is transparent.
This dyed phase change device is useful because it has a good viewing angle, brightness and ON/OFF contrast ratio. It also has a fast turn-OFF time determined by the cholesteric concentration in the liquid crystal material.
Liquid crystal displays may have their electrodes arranged in, e.g., a row and column matrix; one slide has m row strip electrodes and the other slide has n column strip electrodes forming an m.times.n number of intersections. The intersections form elements of a display, in this case an m.times.n element matrix. Each element can be addressed, i.e., have a voltage applied to it, by voltages applied to appropriate row and column electrodes.
A matrix display can be addressed one element at a time, but more often is addressed a line at a time. This is termed multiplexing. A voltage is applied to each row electrode in turn and simultaneously a suitable voltage is applied to selected column electrodes so that all selected element intersections in that row are turned ON. After addressing the whole matrix the addressing is repeated or refreshed for as long as information is to be displayed. The display may also be driven using the correlation properties of the row and column drive waveforms (Nehring, J. Kmetz, A. R., I.E.E.E. Trans. ED, Vol. ED-26 (1979), Pages 795-802). Phase change displays are difficult to matrix or multiplex address because of the time taken for an intersection to turn from ON and OFF. Normally, the turn OFF time is short in comparison to a display refresh time but can be increased by applying a voltage close to a threshold sustaining value to the elements. Threshold voltage is the voltage at which a material reaches its ON state.
Two main types of approaches have been made to multiplexing: a slow and a fast scan method.
In the slow scan method each intersection (element) or line of elements is addressed for a time long enough for the elements to change during their address period. When elements are not being addressed they are sustained in either the ON or OFF state by a voltage close to threshold. This method has the disadvantage of possessing a time lag between a decision being made to change elements and an occurrence of the addressing periods for these elements. Special techniques must also be used in order to turn OFF a previously ON element in a reasonable time.
In the fast scan or RMS method each element or line of elements is addressed for a time much shorter than its response time and this is repeated rapidly so that an RMS voltage above threshold is maintained at ON elements and an RMS voltage below but close to threshold is maintained on OFF elements. This method, when applied to dyed phase change displays, suffers from a long turn OFF time caused by the sustaining effect of the OFF RMS voltage which makes it difficult to change displayed information quickly. Also, the voltage at recently OFF elements causes the dyes to be less absorbing than at elements OFF for longer periods and areas of the display receiving zero voltage, i.e., the areas not covered by electrodes.